The definition of "ultra-high-strength steel" is changing relative to the degree of technological progress required by the times. 

Generally speaking, alloy steel with a yield strength above 1370 MPa (140 kgf/mm) and a tensile strength above 1620 MPa (165 kgf/mm) is called ultra-high strength steel.

According to its alloying degree and microstructure, it is divided into low alloy medium carbon martensitic strengthened ultra-high strength steel, 

medium alloy medium carbon secondary precipitation hardening ultra high strength steel, and high alloy medium carbon Ni-Co type ultra high strength steel. , 

ultra-low carbon martensitic age-hardening ultra-high-strength steel, semi-austenitic precipitation-hardening stainless steel, etc.

Main alloying elements

Ni: is the main forming element of the age-strengthening phase;

it can expand the γ phase area and undergo M transformation (the addition of precipitation strengthening elements is mostly α-forming elements), 

which can maintain good toughness and plasticity of steel and improve the hardenability of steel ( Empty quenching).

Co: can help Ni expand the γ zone, but increasing the Ms point can reduce Ar, which can indirectly improve the strengthening effect.

Ti, Al, Mo, Nb: are age-strengthening elements that can form age-strengthening phases, such as Ni3Al, Ni3Ti, Ni3Mo, (Fe, Ni, Co)2Mo, etc.

Mo: It is also an aging strengthening element, preventing temper brittleness and improving toughness and plasticity.

Among them, C<0.03%; Mn and Si are reduced to a minimum; S<0.01%; other impurity elements such as N, P, etc. must also be strictly controlled.

This kind of steel is generally limited to important components in the aviation, aerospace and weapons manufacturing industries. 

It is only used to produce important components with high specific strength, strong reliability and precise dimensions, 

which are difficult to meet the requirements when using low-alloy ultra-high-strength steel.